I have still more to learn, but I wanted to start a thread in hopes that I can outline my specific situation as I develop a plan to move forward- and hoping from insight from folks who have a lot more practical experience with this stuff. I am sure it's all be done to death 1000 times, but to the newbie their own problem always seems "unique."
I live in a 2 BR condo. I am on the 3rd (and 4th floor) of a 4 floor building. I understand this is not ideal. Moving is not an option. I have one other unit directly beneath me. I am a trumpet player.
My wish list: I would like to be able to practice at home. #1 goal (and the only critical one.)
Bonus goals: 1a) have acceptable acoustics in the room to record remote tracks (I realize a small room is not the ideal for recording trumpet, but it would be a bonus if it would work for commercial remote tracks- not talking about a CD for Telarc.) 2a) if It was a large enough space to teach another student. This is not a major issue, but if the space was that size it would also be more comfortable to play in.
I am in the planning stages- but am ready to build immediately once I have a workable plan that I am confident will meet my objectives.
The trumpet can be pretty loud- and I can be a loud trumpet player. But I don't need to play at the EXTREME levels very much or very often. Likely would save that for when no-one is around. But I would say range is 80-100.
I have a larger space (still small) which is a walk-in closet (on the 3rd floor.) It does not touch any other neighbors (no shared walls, just someone below.) There is a neighbor directly below. This room is already enclosed and the dimensions of the existing room are about 6 feet x 9.5 feet with 8 foot ceilings. The walls to this room are 1 sheet drywall, studs with insulation, and 1 sheet drywall. The existing floor is carpet, pad, 2 layers of subfloor, insulation as well as air around the engineered beams supporting floor and then the drywall on neighbors ceiling.
My initial plan to was build a "room within a room"- lift the floor up on a riser of some sort TBD- basically whatever would give the best noise reduction for downstairs neighbor... and then stud walls and ceiling with 2 xdrywall and green glue/ insulation/ and 2 x drywall green glue box with everything about 2" away from existing walls. I would put a solid door with mass added and seals, as well as solid door into the existing closet with seals. I would add isolated vents w/ fan.
Budget- well, I am willing to spend to do it right, however I would say my budget will be determined by the limitations of my location. I am not re-engineering the building. I am willing to do what it takes for the best isolation within the parameters of keeping it "inside my unit" and not exceeding the existing structural limitations of the building. We plan to be here for a long time, and it's worth it to invest to me- and I think we are likely to be more limited by the existing structure than by our budget.
My cousin is a structural engineer (conveniently) and he plans to help me make certain that I have everything in line give the construction- I have the building plan, and once I have an idea of what I would like to do he will tell me if I CAN do it

It is worth pointing out- trumpet does not have a lot of low end, and can't even reach frequencies below 160 HZ, so this is worth pointing out. It might be nice to play music in the room to play along with, but for certain, there would not be any kind of low end like people talk about in their studio or theater rooms.
My question is:
While I have a lot to learn and understand about everything- the floor seems to be the "trickiest" part given that my only major concern is downstairs neighbor- since the rooms only "shared" walls are within my unit. Basically, I feel confident that what I do will quiet it enough to work for my wife within our unit or for other units not directly connected to the practice room- but my downstairs neighbor is the key. In order for me to feel comfortable, I would need to feel very confident that my playing at normal hours would not be a distraction/disturbance in her unit. Give the reasonable floor options (i.e. not pouring a concrete floor in my unit) is this a reasonable possibility? Or are my hopes too high? A little insight will be super helpful.
Thanks-! ( I need to do a drawing... I'll get there!!)